Only one built (converted 320C) to test validity of high-bypass turbofans on the airframe. Cost of conversion to new engines found unjustified for new production aircraft. 152’11” length. 142’8.5″ span. 12’4″ inches fuselage width. CFG-56 turbofans.

707-520

Reengined 120

Reengined 120 with RR Conway 10 engines.

707-520B

320B stretch

2′ stretch of the 320B with JT3D-5A turbofans.

707-620

120 Stretch

10′ stretch of the 120 with Bristol 511 engines.

707-720

320 with IGW

Increased Gross Weight variant of the 320 with Bristol 511 engines.

707-320-101

320 with enlarged lower lobe

320 with an enlarged lower lobe for passenger seating on a lower deck forward of the wing and more cargo aft of the main gear.

707-520-X

Double decker development

Double decker with low wing. Length 191’7″.

707-520-2X

520-X shrink

Double decker with low wing. Shorter than the above.

707-520-X3

Double decker development

Double decker with high wing. Length 144’2″. Span 142’5″.

735 and 738

Swing tail cargo variants

735 had an upward swinging tail and 738 a sideways swinging one. Both enabled cargo loading through the resultant opening.